Even if they won't admit it, I'm sure some people open their inbox and stare blankly as a subconscious way of taking a break.
Plus, some people do generally have answering email as their job.
Lets face it we tend to do those things through Web interfaces on desktop/laptops or in messaging style apps on phones that tend to look quite similar in layout.
And we need to be clear about the time frame as other posters have mentioned (per day or per week)
On the work based figures, I suspect that people include the time spent on a task generated by the email as 'doing email'. My colleagues certainly would, and a lot of the work related email we get is providing information directly into a rich format email.
This probably accounts for a large chunk of the reported hours, as well.
I lost my window seat a while ago and that's one thing I really miss. I used to look out the window and think. Now I often obsessively check Email or Hacker News.